a. Field of the Invention
This invention relates to a system and method for assessing the force and torque between an electrode and tissue in a body. In particular, the instant invention relates to a system and method for assessing the force and torque between an electrode tip on a diagnostic and/or therapeutic medical device such as a mapping or ablation catheter and tissue, such as cardiac tissue. The instant invention also relates to a method for sensing and calculating contact force exerted by another component on a tissue, and generally, a method for sensing and calculating contact force on an elongate member when in contact with another component or structure, for medical or non-medical purposes.
b. Background Art
Electrodes are used on a variety of diagnostic and/or therapeutic medical devices. For example, electrodes may be used on cardiac mapping catheters to generate an image of the internal geometry of a heart and electrical potentials within the tissue. Electrodes are also used on ablation catheters to create tissue necrosis in cardiac tissue to correct conditions such as atrial arrhythmia (including, but not limited to, ectopic atrial tachycardia, atrial fibrillation, and atrial flutter). Arrhythmia can create a variety of dangerous conditions including irregular heart rates, loss of synchronous atrioventricular contractions and stasis of blood flow which can lead to a variety of ailments and even death. It is believed that the primary cause of atrial arrhythmia is stray electrical signals within the left or right atrium of the heart. The ablation catheter imparts ablative energy (e.g., radiofrequency energy, cryoablation, lasers, chemicals, high-intensity focused ultrasound, etc.) to cardiac tissue to create a lesion in the cardiac tissue. This lesion disrupts undesirable electrical pathways and thereby limits or prevents stray electrical signals that lead to arrhythmias.
The safety and effectiveness of many of diagnostic and/or therapeutic devices is often determined in part by the proximity of the device and the electrodes to the target tissue. In mapping catheters, the distance between the electrodes and the target tissue affects the strength of the electrical signal and the identity of the mapping location. The safety and effectiveness of ablation lesions is determined in part by the proximity of the ablation electrode to target tissue and the effective application of energy to that tissue. If the electrode is too far from the tissue or has insufficient contact with the tissue, the lesions created may not be effective. On the other hand, if the catheter tip containing the electrode contacts the tissue with excessive force, the catheter tip may perforate or otherwise damage the tissue (e.g., by overheating). Therefore, to successfully ablate live tissue, the electrode should be applied to the tissue with proper force. When ablating and moving an electrode, in addition to the magnitude of the force, knowledge of direction of the force (i.e. multi-axial measurement) and further the torque acting on the electrode tip are important for estimating the distribution of pressure and stress over an electrode tip surface.
Contact force between a catheter electrode and tissue has typically been determined using one or more of the following methods: clinician sense, fluoroscopic imaging, intracardiac echo (ICE), atrial electrograms (typically bipolar D-2), pacing thresholds, evaluation of lesion size at necropsy and measurement of temperature change at the energy delivery site. Each of these methods has disadvantages, however.
For example, although a clinician can evaluate contact force based on tactile feedback from the catheter and prior experience, the determination depends largely on the experience of the clinician and is also subject to change based on variations in the mechanical properties of catheters used by the clinician. The determination is particularly difficult when using catheters that are relatively long (such as those used to enter the left atrium of the heart).
Because fluoroscopic images are two-dimensional projections and blood and myocardium attenuate x-rays similarly, it can be difficult to quantify the degree of contact force and detect when the catheter tip is not in contact with the tissue.
Intracardiac echo can be time consuming and it can be difficult to align the echo beam with the ablation catheter. Further, intracardiac echo does not always permit the clinician to confidently assess the degree of contact and can generate unacceptable levels of false positives and false negatives in assessing whether the electrode is in contact with tissue.
Atrial electrograms do not always correlate well to tissue contact and are also prone to false negatives and positives. Pacing thresholds also do not always correlate well with tissue contact and pacing thresholds can be time-consuming and also prone to false positives and negatives because tissue excitability may vary in hearts with arrhythmia. Evaluating lesion size at necropsy is seldom available in human subjects, provides limited information (few data points) and, further, it is often difficult to evaluate the depth and volume of lesions in the left and right atria. Finally, temperature measurements provide limited information (few data points) and can be difficult to evaluate in the case of irrigated catheters.
The inventors herein have thus recognized a need for a system and method for determining the contact force and torque upon an electrode tip, both during RF ablation and when driving the RF electrode to the ablation site, that will minimize and/or eliminate one or more of the above-identified deficiencies.